For the regional measurement of the pressure-volume relationship the so-called electrical impedance tomography is known. In this process, a number of electrodes are placed around the thorax, wherein an alternating current with e.g. 50 kHz at 5 nA peak to peak amplitude is applied to respectively adjacent electrodes. The other electrodes respectively are used with the alternating current to carry out the measurement of impedance against a defined reference potential. As soon as all the electrodes, one after another, have served as current conducting electrodes, a cycle for data detection is concluded. In order to eliminate statistical disturbances, as a rule a plurality of data detection cycles is averaged, in order to obtain a corresponding picture. The maximal impedance changes in the zone of the thorax are caused by the breathing in and out of air. In this context it can be observed that the impedance change which is measured by electrodes is a measure of the change of volume in the lung. Therefore according to the process of electrical impedance tomography, measurements can also be carried out with respect to the pressure-volume relationship in the lung. However, the special feature of electrical impedance tomography is that on the basis of a computer-based evaluation of the signals at the electrodes, a two-dimensional or even three-dimensional image of the impedance changes can be compiled.
From Dijkstra A. M. et al.: “Review Clinical Applications of Electrical Impedance Tomography”, Journal of Medical Engineering & Technology, G B, Basingstoke, Hants, no. 3, May 1993 (1993–05), pages 89–98 a general review of clinical applications of electrical impedance tomography is known. It is shown that besides respiratory applications also applications for the central nervous, cardiovascular and digestive systems are possible.
From Eung Je Woo et al.: “Measuring Lung Resistivity Using Electrical Impedance Tomography”, IEEE Transactions on Biomedical Engineering, US, IEEE Inc. New York, vol. 39, no. 7, 1 Jul. 1992 (1992-07-01), pages 756–760 a method for measuring the lung resistivity using electrical impedance tomography is known. It is proposed to use the electrical impedance tomography imaging techniques in the measurement of lung resistivity for the detection and monitoring of apnea and edema.
The artificial respiration of a sick lung, wherein oedemas have formed, is a special problem, because it cannot be exactly controlled whether the lung has already closed and/or collapsed in certain parts. Then it was found that the mortality rate can be reduced substantially when a predetermined pressure is artificially maintained in the lung, which just makes possible keeping open all the alveoli (terminal lung units, air sacks). However, this pressure is not known in the case of artificial respiration, because the alveolar opening and/or closing of the lung could not yet be regionally determined.
Therefore the object of the invention is to make available a method for the determination of the alveolar opening and alveolar closing of the lung, depending on the respiration pressure.
This object is solved by a method comprising the features according to claim 1 and by an apparatus comprising the features according to claim 25. The method according to the invention is based on the cognition that the alveolar opening and/or closing can be determined from an impedance signal gained with the method of electrical impedance tomography. Thereby at least two important values can be determined, namely a first respiration pressure value which corresponds to the alveolar closing of the corresponding lung zone and a second respiration pressure value which corresponds to the alveolar opening of the corresponding lung zone.
Accordingly, the apparatus according to the invention comprises a means for measuring according to the method of electrical impedance tomography an impedance signal (AU) in at least one lung zone depending on the respiration pressure, a means for determining from the impedance signal a first respiration pressure value which corresponds to the alveolar closing of the corresponding lung zone, and a means for determining from the impedance signal a second respiration pressure value which corresponds to the alveolar opening of the corresponding lung zone.
In contrast to computer tomography and magnetic resonance tomography, the process according to the invention can also be carried out at the bed of the patient, because no costly instruments are necessary. In this case there is no radiation stress either for the patient or for the staff. In the case of critical patients constant supervision of the state and degree of openness of the lung can therefore be carried out.
The first effect of the process according to the invention is that the impedance signal is influenced by the breathing movements of the patient. In each breathing movement the lung volume rises and falls. Using the regional impedance curves of electrical impedance tomography it can be observed that the average change of the impedance signal, due to breathing movements, is conspicuously greater in zones wherein the lung has not yet collapsed, whereas in zones wherein the lung has already collapsed, only minor changes in the impedance signal are caused. For example the change in the impedance signal due to breathing movements can be determined on the basis of the unaveraged root mean square of the impedance signal over a plurality of breaths. The change in the impedance signal on the basis of breathing movements is therefore determined from the signal energy of the high frequency portions of the impedance signal, which are based on the breathing movements. But it is equally possible that the change in the impedance signal based on breathing movements can be determined on the basis of an average peak to peak value of the impedance signal over a plurality of breaths.
The alveolar closing and/or opening of the lung or the first and second respiration pressure value respectively is determined on the basis of the change in the impedance signal due to breathing movements, in that the change in the impedance signal based on breathing movements is compared with predetermined breathing movement comparative values. In doing so, it must be taken into account that with respect to the two comparative values, as a rule a certain hysteresis is found. This means that the opening of the pulmonary cells does not take place at the same pressure as the closing of the alveoli (terminal lung units), but that both comparative values fall away from each other. In this context it must in addition be taken into consideration in which direction the respective comparative value passes in order to be able to precisely identify the hysteresis.
With respect to the comparative values it is conceivable that fixed comparative values are predetermined. However, in this case disturbance factors, e.g. based on offset changes, enter fully into the measurement. Therefore it is expedient to determine the breathing movement comparative values dynamically from the average change in the impedance signal on the basis of breathing movements of another zone of the lung. Preferably the lung is divided into a plurality of zone planes perpendicularly to the gravity vector, wherein the other lung zone is a zone which is in the direction of the gravity vector above the lung zone which is concerned. In this case use is made of the fact that as a rule the lung part which is lowest in the direction of the gravity vector is more strongly affected by the pathological appearance of the collapse of the alveoli (terminal lung units) than the correspondingly higher part of the lung zone. Alveolar closing of a lung zone, for example, can be determined as soon as the breathing movement comparative value of the lower lung zone is less by a predetermined factor than the breathing movement comparative value of the lower zone.
A further effect which is suitable to determine the alveolar opening or closing of the lung or the first and second respiration pressure value respectively is the change in the impedance signal due to the collapse of the alveoli. In the case of a pathological lung or an unphysiological condition such as i.e. anaesthesia it is observed that even with constant pressure the lung zone collapses, i.e. the pulmonary units therefore collapse spontaneously. This collapse takes place all the more strongly as the respiration pressure falls, wherein the effect in addition is reinforced like an avalanche over time. Consequently according to the invention alveolar closing of the lung zone or the first respiration pressure value respectively is determined as soon as the average change in the impedance signal due to the collapse of the alveoli falls below a collapse comparative value. Accordingly alveolar opening of a lung zone or the second respiration pressure value respectively is found as soon as the average change in the impedance signal based on the opening of the alveoli is above an opening comparative value.
The average change in the impedance signal due to the collapse of the alveoli, for example, can be determined on the basis of the mean increase in the impedance signal depending on time with a predetermined respiration pressure.
The average increase, for example, can be determined by the Gauβ compensation computation, in that a straight line is placed in a segment of the impedance signal depending on time at constant pressure. The collapse comparative value and/or the opening comparative value can be prescribed as fixed values, or however they can be determined from a dynamic comparative value determination. The dynamic determination of the comparative value is carried out expediently on the basis of an impedance signal in a different lung zone. Preferably the lung is divided, as was described above, into a plurality of zone planes in the direction of the gravity vector, wherein the comparative value is derived from the lung zone which is above the lung zone concerned in the direction of the gravity vector.
A further effect caused by the alveolar opening or closing of a lung zone is the average change of the impedance signal on the basis of respiration pressure changes. As soon as a sudden respiration pressure change is applied to the lung, the impedance signal for this pressure change does not follow at once, but respectively with a certain delay.
Accordingly, alveolar closing or the first respiration pressure value respectively of a lung zone is determined, as soon as the average change in the impedance signal based on respiration pressure changes falls below a first respiration pressure comparative value, and wherein an alveolar opening or the second respiration pressure value respectively of a lung zone is determined as soon as the average change of the impedance signal based on respiration pressure changes moves above a fixed second respiration change comparative value. In this context use is made of the observation that the lung mechanics responds with a certain inertia to changes in pressure. This inertia is larger in the sick zones than in the healthy zones of the lung, because the sick zones only open as from a higher pressure, so that the sick zones can be localised according to the invention.
The change in the impedance signal due to respiration pressure changes, for example, can be determined on the basis of the average initial rise in the impedance signal after a sudden increase in respiration pressure. The initial rise is all the smaller, the more the lung zone which is concerned tends on the basis of pathological changes to a collapse of the terminal lung units or alveoli. Another possibility is that the change of the impedance signal on the basis of respiration pressure changes is determined based on the time constant of the impedance signal, with which the impedance signal follows a change in the respiration pressure. The first respiration pressure comparative value and/or the second respiration pressure comparative value can be prescribed or, however, can be determined dynamically, as was described already above for the other processes. In the case of dynamic determination of the first respiration pressure comparative value and/or of the second respiration pressure comparative value, the determination is carried out on the basis of the average change of the impedance signal due to respiration pressure changes in another lung zone. The other lung zone is again preferably a zone which is in the direction of the gravity vector above the lung zone concerned. In this process the lung is subdivided for the measurement into a plurality of zone planes in the direction of the gravity vector.
According to a preferred embodiment it is provided that setting out from a respiration pressure wherein the lung alveoli are opened in almost all the lung zones, the respiration pressure is reduced step by step, until an alveolar closing of a lung zone is found in one lung zone.
Apart from the division of the lung into zones in the direction of the gravity vector, it is also conceivable that the lung is divided into a plurality of radial sectors, wherein the centre point axis of the sectors is in the direction of the gravity vector.
A device for carrying out the method according to the invention consists of a plurality of electrodes which are applied around the thorax, of an electrical impedance tomograph for the control of individual electrodes and for the evaluation of the impedance signals at the uncontrolled electrodes, in order to obtain a regional impedance signal in the thorax, and of a processing unit to evaluate the regional impedance signals for determining the first respiration pressure value and the second respiration pressure value. Falsification of the signals is to be determined in this context, in particular, due to breathing movements, because on each intake or outlet of breath, the positions of the electrodes in relation to each other alter. In order to eliminate the resultant signal falsifications at the electrodes, a sensor is provided to measure the changing periphery of the thorax caused by the breathing movements. In addition, the electric impedance tomograph comprises a correction unit, wherein the change of impedance signals of the electrodes caused by breathing movements is corrected by including the sensor signal.
An important aspect of the apparatus according to the invention is to control an artificial respiration unit. This can be particularly useful for a sick lung because it cannot be exactly controlled whether the lung has already closed and/or collapsed in certain parts. However, according to the invention it was found that the mortality rate can be reduced substantially when a predetermined pressure is artificially maintained in the lung, which just makes it possible to keep open all the alveoli. This can be done by providing a control unit which is connected to the artificial respiration unit and the processing unit, whereby the first respiration pressure value and the second respiration pressure value is fed from the processing unit to the control unit to control the artificial respiration.
The signals obtained by regional impedance tomography can be used to determine an optimal therapeutic level of the so-called positive end-expiratory pressure (PEEP). It is important to find an optimal biological compromise between treating alveolar overdistension in one part of the lung and atelectasis in another. As a priority, PEEP levels must be set high enough to prevent as much as possible the collapse of alveoli at the end of expiration in the most dependent parts of the lung; at the same time the over-stretching of the non-dependent upper parts on the lungs must be avoided. Both these pathological conditions—alveolar collapse and alveolar overdistension—can be recognized as a reduced amplitude of the ventilation-induced impedance changes in a regions of interest. An optimal level of PEEP, however, leads to an even distribution of ventilation (and thus impedance changes) throughout the entire lung.
In addition, an optimal level of PEEP prevents the collapse of airways. If airways are kept open during the entire respiratory cycle, the respiratory gases are exchanged efficiently. These parts are thus ventilated and the impedance signals follow this ventilation. If, however, the conducting airways are collapsed during the entire respiratory cycle, the terminal lung units—in particular the alveoli—are cut off from the supply of fresh gas. Gas exchange suffers and no ventilation-induced change in the impedance signal can be detected. These lung areas become silent on the impedance tomographic image. The oxygen within the cut-off alveoli is absorbed and with the progressive decrease in their gas content, the absolute impedance of such a lung unit is reduced. In a scenario where PEEP levels are not high enough to prevent the expiratory collapse of airways and terminal lung units (alveoli) but where pressures are sufficiently high to open collapsed airways during inspiration, ventilation of these lung units takes place only during this period of the respiratory cycle. The changes in the impedance signals of such a lung region can be amplified compared to an area of normal ventilation since these collapsed lung units start from a low expiratory air content but are filled rapidly to approximately normal volumes during inspiration. During expiration they collapse, again and the process of tidal recruitment/collapse begins anew.
Observing the signals from regional impedance tomography it is possible to determine the points of airway/alveolar opening and closing by systematically titration inspiratory and expiratory airway pressures.
In accordance with a further aspect of the present invention, the apparatus comprises a monitoring unit for monitoring the first respiration pressure value and the second respiration pressure value. By monitoring these values the patient can be observed by a monitoring device gaining important pieces of information with regard to the lung functioning. All the direct and derived impedance signals and/or images discussed above should be calculated continuously and should be available for on-line display. Any single one of them or a combination of them can be used for the automatic or semi-automatic control of a therapeutic device, such as a mechanical ventilator. The information obtained by electrical impedance tomography can be used to guide specific clinical maneuvers aiming at optimal lung recruitment and at keeping most alveoli open or at finding the best biological compromise between alveolar over-distension and alveolar collapse.
Furthermore, regional pressure-volume curves generated by electrical impedance tomography can be used to define pressure points of specific clinical relevance. These points are the alveolar opening and closing pressure of a specific lung region, the lower and the upper inflection point of the inspiratory and the expiratory pressure-volume curve. Additional information on lung behavior can be obtained by analyzing the shape and the area the pressure-volume-curve.
Further details and advantages of the invention will be explained in more detail on the basis of the example of an embodiment shown in the drawing. It shows: